One day 40 years ago, Maestro George Cleve and a few of his musician friends were kicking back with a few beers during a break from rehearsals for the San Francisco Spring Opera’s production of Mozart’s “The Abduction From The Seraglio.”

“This really is the best music, isn’t it?” said one.

“Yeah,” said another. “Wouldn’t it be great if we could play nothing but Mozart?”

“Let’s have an all-Mozart festival!” said a third.

“Well,” said Cleve, “I’m not very good at organizing these things, but if you organize it and want me to conduct it, let me know.”

And that was the end of that — or so he thought. But a couple of months later they called him and said, “OK, it’s organized.”

Thus was born the Midsummer Mozart Festival, the only all-Mozart festival in North America. There are a lot of “Mostly Mozart” festivals, but this is the only one that’s all Mozart, all the time.

That first year, the musicians split the proceeds among themselves. It came to about 10 bucks apiece. This year, as the festival celebrates its 40th birthday, it has grown to the point at which the musicians are paid union scale. But to tell the truth, they’re not in it for the money. They’re in it for the Mozart.

“He’s the best,” says violinist Robin Hansen, who is celebrating an anniversary of her own — her 20th year as the orchestra’s concertmaster. “No one else can delight your senses while at the same time touching the deepest places in your heart. I often find myself with a smile on my face and a tear in my eye, both at the same time. He’s so much fun to play!”

And to listen to. You can’t ask for anything more fun than the overtures to “The Marriage of Figaro” and “The Abduction From The Seraglio.” Or more majestic than the great Symphony No. 40 in G minor. Or more delightful than the Divertimento for flute, oboe, bassoon, four horns and strings, which he wrote when he was only 16.

All these, and a lot more, will be featured at this year’s festival, which will run from July 20-27 with two completely different programs.

The first concert — July 20 at Stanford’s Bing Concert Hall and July 21 at First Congregational Church in Berkeley — will feature the 40th Symphony, the overture to “Figaro,” and two arias sung by mezzo-soprano Tania Mandzy.

The second concert — July 25 at St. Marks’s Lutheran Church in San Francisco, July 26 at First Congregational Church in Berkeley and July 27 at Bing Concert Hall at Stanford — will feature Seymour Lipkin playing the Piano Concerto in D Minor, mezzo-soprano Anna Yelizarova singing “Parto, parto” from “La Clemenza di Tito,” and the San Francisco Boys Chorus singing the delightful Spatzen Mass (“Sparrow Mass”).

All the performers are world-class musicians, but the real draw is Cleve himself.

“Definitely the highlight of my career is playing Mozart with George,” says Hansen. “I’ve been fortunate enough to work with Leonard Bernstein, Herbert Blomstedt, Zubin Mehta and Michael Tilson Thomas. George is one of those musical giants. You’re very lucky if you get to work with someone of his caliber in your musical career.”